The present invention relates to structure for controlling feedback in a circulation control pneumatic valve and more particularly to means for mounting the nozzles of air ducts which carry air to a rotor blade.
Presently, maximum useable airspeed in helicopters is limited because of loss of lift and other problems encountered with helicopter rotors at high speeds. At any given instant, one or more rotor blades, referred to as "the advancing blade", are in the part of their rotation cycle where the effective airspeed experienced is the sum of their rotational velocity and the helicopter's forward airspeed. At the same time, the blade or blades mounted on the rotor opposite the advancing blade experience an effective airspeed equal to their rotational velocity less the forward airspeed of the helicopter. These blades are referred to as "the retreating blade". It will be appreciated that as helicopter forward airspeed increases, the effective airspeed at the advancing blade increases, while the effective airspeed at the retreating blade decreases. Since the contribution to total effective airspeed of the rotor blade due to the blade's rotational velocity is a function of the radial distance from the rotor hub, the point on the rotor experiencing the highest total effective airspeed is at the tip of the advancing blade, while the lowest total effective airspeed will be found near the root of the retreating blade. Both of these effects lead to problems at high speeds.
The main problem associated with the rotor when the helicopter is at high airspeed involves loss of lift on the retreating rotor blade, due to low effective airspeed and high angle of attack. The angle of attack of the retreating blade is increased at high helicopter airspeed because the induced velocity becomes large in relation to the oncoming free stream velocity. When the angle of attack reaches about 14.degree. the airfoil section stalls and the lift is appreciably reduced. The low effective airspeed experienced by the retreating blade when the helicopter is at high airspeed also contributes to loss of lift. At still higher helicopter airspeeds a portion of the retreating blade near the rotor hub actually experiences reverse flow, i.e., airflow from trailing edge to leading edge, because the helicopter forward airspeed exceeds the product of rotational velocity times distance from the rotor hub. This region of reverse flow extends further outward on the retreating blade as helicopter airspeed continues to increase. Heretofore, this region has been relatively useless for producing lift, and in fact has contributed to much buffeting and vibration of the rotor blade.
One conventional means for altering the lift of helicopter blades is to cyclically alter the blade angle of attack and the blade speed. This has been accomplished by mechanical means.
A second method employs a circulation control rotor blade which carries air for circulation control blowing. A number of slots are provided in the blade and, as the blade is rotated, air is blown out these slots in a thin sheet. The thin sheet adheres to the trailing edge and remains attached, by the Coanda effect, until it reaches the separation point on the blade under side, beneath the trailing edge. The point of separation beneath the trailing edge is determined by the intensity of blowing. The effect of the circulation control is to relocate the stagnation stream lines and produce a higher lift on the foil, the lift on the airfoil being functionally related to the ratio of the velocity of the blown air to the free stream velocity blowing over the rotating wing.
A fundamental part of the circulation control rotor concept is a pneumatic control valve which controls the distribution of the circulation control airflow around the rotor disc. Collective control is achieved by supplying a uniform airflow through all blades simultaneously, and cyclic control is obtained by modulating the airflow to each blade azimuthally, once per revolution.
One type of valve being used experimentally on military aircraft is a flex ring valve which uses a flat flexible ring located below and adjacent to the plane of the nozzles. The flexible ring is supported from beneath by an array of actuators of relatively short stroke. One disadvantage to the aforementioned flex ring valve is the occurrence of unwanted levels of feedback control input from sources other than the control system itself. For example, consider the case in which a valve has commanded a collective increase in the lift of all blades of the circulation control rotor to initiate a rate of climb for a change in cruise altitude. To accomplish this change, a control linkage system for the valve would displace the entire valve assembly and its supporting ring and central pivot downwards, thereby increasing the gap between the flexible ring valve plate and the blade nozzle. With the larger entry area thus exposed, the pressure drop through the flow constriction at the gap would decrease and the flow through the valve increase, thereby raising the pressure in the blade duct. The increased flow from the blade duct through the circulation control blowing slot near the blade trailing edge would then induce the desired lift increase from the rotor blades. The increased rotor thrust would cause the rotor hub to rise, minutely, due to the small but significant elastic deformations of the hub itself, the rotation bearings and the stationary mast/plenum. This upwards displacement of the hub would carry with it the blade duct nozzles, but since the value assembly is mounted, effectively, from the lower porton of the stationary mast, the valve assembly would not experience a comparable upward displacement and thus the air gap would increase. An increase in the air gap will produce an increase in blade lift and this lift change would tend to further open the air gap. Such a condition is unstable in character since the response to control input results in a positive feedback and causes an overshoot of the desired output. This overshoot opens the possibility that unstable oscillations could occur.